Lubricating oils used in internal combustion engines are subject to deterioration in the presence of oxygen. Oxidation of these oils eventually leads to the formation of sludge and varnish materials which deposit upon the various engine parts. These deposits result in ring sticking, poor heat radiation, and reduced lubrication which causes accelerated wear and eventual engine failure. Attempts to combat these problems include the addition of antiwear and antioxidant agents to the oil. Lincoln, in U.S. Pat. No. 2,441,496, teaches that a combination of sulfurized monomer olefins and organic phosphorus compounds, when added to lubricating oils, act to limit oxidation and corrosion as well as increase the film strength of the oil. Increased film strength of the oils allows smaller bearing areas to support the same or heavier loads without the danger of the oil being squeezed from between the rubbing surfaces. Makeska, in U.S. Pat. No. 2,443,264 employs organic compounds containing both phophorus and sulfur in mineral lubricating oils as inhibitors of oxidation and as agents for promoting engine cleanliness generally. Crosby et al, in U.S. Pat. No. 2,983,681, disclose lubricating oil compositions containing a combination of sulfurized isoprenoid compounds and organophosphorus, organoarsenic or organoantimony compounds, which are added to provide the lubricant with improved antiwear properties and oxidation stability. Additionally, Colclough et al, in U.S. Pat. No. 3,687,848, incorporate into a lubricating oil, an antioxidant and antiwear additive mixture of a particular phosphorothionyl or phosphonyl sulphide with an organic ammonium thiophosphate.
Presently, one of the most commonly employed lubricating oil additives is zinc dialkyldithiophosphate (ZDTP). This multifunctional lubricant additive was initially added to automobile lubricants as an antioxidant, but now is more widely used as an antiwear additive. However, recent studies have indicated that the combined presence of both zinc and phosphorus in automobile exhaust, which results from the decomposition and use of ZDTP, decreases the longevity of catalytic converters on automobiles.